libstd: set baseline stability levels.

Earlier commits have established a baseline of `experimental` stability
for all crates under the facade (so their contents are considered
experimental within libstd). Since `experimental` is `allow` by
default, we should use the same baseline stability for libstd itself.

This commit adds `experimental` tags to all of the modules defined in
`std`, and `unstable` to `std` itself.

src/libstd/ascii.rs

@@ -10,6 +10,8 @@
 
 //! Operations on ASCII strings and characters
 
+#![experimental]
+
 use collections::Collection;
 use fmt;
 use iter::Iterator;